The present invention relates to a line thermal head and more particularly to a method of transferring printing data for a line thermal head main body.
A line thermal head has a heating array wherein a plurality of heating elements each comprising a resistor are arranged in a line. It performs printing of a line by selectively applying a driving current of several tens mA to the resistor of each heating element to cause it to heat up, thereby causing color development on a thermosensible paper, or by melting the ink on a thermal-transfer ribbon to be transferred onto a plain paper. Since a number of heating elements is included in the heating array of a line thermal head, i.e., the number of dots per line is extremely large, if all heating elements are driven at a time, a power source having a heavy current capacity must be used. To avoid this, in a normal line thermal head, a heating array constituting one line is divided into a plurality of physical blocks, and time division driving is performed on a block basis. This allows the quantity of current consumed in one time division driving operation to be reduced and, therefore, the capacity of the power source can be reduced to some extent. If there are too many divisions, however, the writing between a head main body portion and a head control portion becomes complicated, resulting in an increase in the number of signal lines. For this reason, the linear heating array is conventionally divided into only a few blocks. As a result, the number of dots per one physical block is still considerably large.
A brief description will now be made of one a method of transferring printing data on a line basis to a line thermal head main body portion having such divided physical blocks. First, an exponent n is set to 0 at step S1 as shown in the flow chart in FIG. 5. The exponent n indicates a number assigned to each physical block. Next, a head data counter is cleared at step S2. This counter is for counting the number of dots to be printed. Then, the number of the bytes (HBYTERBL[n]) of printing data to be transferred to the nth physical block specified is loaded at step S3. Further, printing data for HBYTERBL[n] bytes is transferred to the head main body at step S4. At step S5, the value counted by the head data counter or a dot counter is stored in a specified area HDOTBL[n] of the control portion. Thus, when the printing data is transferred to the specified nth physical block, the number of dots to be printed is recorded at the same time for that physical block. Next, the exponent n is updated to n+1 at step S6. Thereafter, the process returns to step S2 to transfer printing data for the (n+1)th physical block and record the number of dots to be printed. Thus, transfer of printing data is sequentially performed for each physical block.
A conventional method of driving a line thermal head will now be briefly described with reference to the flow chart in FIG. 6. First, printing data is transferred to a head main body portion at step S1. This transfer method is as shown in the flow chart in FIG. 5. Next, a driving pattern of the line thermal head is decided at step S2. The driving pattern means the timing for the application of a current to each physical block. Specifically, the timing for the application of a current to each physical block is set in accordance with the number of dots to be printed recorded at step S5 in the flow chart shown in FIG. 5. When the total number of dots to be printed, i.e., the total number of the heating elements to which a current is to be supplied is large, each physical block is driven on a time division basis and, conversely, when the number is small they are driven at a time. At step S3, the line thermal head is driven to perform printing in accordance with the driving pattern thus set.
As described above, in the conventional method of transferring printing data, printing data for one line is simply supplied to the head main body portion for every transfer process in order to perform high speed printing using simple transfer control. Therefore, when line printing is performed in accordance with the printing data which has been transferred, even if the time division driving is sequentially performed for each physical block, the maximum number of-dots printed in one driving process is equal to the number of heating elements included in a physical block. That is to say that the conventional method does not allow the maximum number of dots printed in one driving process to be set to a value which is smaller than the number of heating elements included in a physical block (the largest physical block when the physical blocks vary in size).